The present invention relates generally to radar systems and more specifically the invention pertains to a system which produces low sidelobe levels in reflector antennas. In radar systems, the system will suppress interference, using a reflective antenna with a resistive taper that generates desired bistatic scattering and backscattering patterns. Antenna synthesis techniques that relate the scattered field to the induced surface current density to get low sidelobes and nulls in the scattering patterns are used to design the resistive taper for different applications.
Scattering occurs when an electromagnetic wave impinges on an object and creates currents in that object which reradiate other electromagnetic waves. The electromagnetic wave may be of any frequency, but most of our every day encounters with scattering involve light. As technology advances, however, scattering from invisible spectrum, particularly microwaves, becomes more and more important. Public concerns involving the impact of microwaves on the environment and health, and military concerns involving very low sidelobe antennas and targets with a low radar cross section (RCS) point to a need for controlling the scattering of electromagnetic waves at microwave frequencies.
Current methods for constructing low sidelobe reflectors for radar systems include: phased array feeds, rim loading, shaping the reflector, and using subreflectors. Phased array feeds provide greater control over the sidelobe levels of the reflector, but are very expensive and large. For rim loading, constant resistive and impedance edge loads are placed on the rims of the reflector to reduce large current spikes at the edges of the reflector. Since the rim loads are a constant resistivity they provide only a limited control of the sidelobe level and lower, but don't eliminate, the current spikes at the edges.
Shaping the reflector entails rolling the edges of the reflector to help lower the sidelobe level. This does not provide a taper to the current density to produce very low sidelobes.
Finally, the use of subreflectors does reduce the blockage of the radiation, but this technique only provides limited control over the sidelobe levels.
The practice of rim loading reflector antennas to provide control over the performance characters of the antennas has been discussed in two articles by Ovidio Bucci et al:
Ovidio M. Bucci, et al., "Control of reflector antennas performance by rim loading," IEEE Trans. Antennas Propagat., vol. AP-29, no. 5, Sep 1981, pp. 773-779; and
O.M. Bucci and G. Franceschetti, "Rim loaded reflector antennas," IEEE Trans. Antennas Propagt., vol. AP-28, no. 3, 1980, pp. 279-305. The disclosure of these articles is incorporated by reference, since they relate antenna surface impedance boundary conditions to the antenna's performance.
The task of reducing sidelobes is also alleviated, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 3,314,071 issued to Lader; PA0 U.S. Pat. No. 3,156,917 issued to Parmeggiani; PA0 U.S. Pat. No. 4,376,940 issued to Miedema; and PA0 U.S. Pat. No. 4,642,645 issued to Haupt.
Currently, three primary methods exist to reduce microwave scattering from an object: covering it with an absorber, changing its shape, and detuning it through impedance loading. Absorbers convert unwanted electromagnetic energy into heat. An example of absorption is lining an anechoic chamber with absorbers. Changing the shape of the object channels energy from one direction to another, changes dominant scattering centers, or causes returns from various parts to coherently add and cancel the total return. Examples include rounding sharp edges, making an antenna conformal to the surface of an airplane, and serating the edges of a compact range reflector. Impedance loading alters the resonant frequency of an object. Examples include making a radome transparent to signals in the frequency band of the antenna and detuning the support wires of a broadcast antenna. Often, a combination of these techniques is necessary to reduce the scattering to an acceptable level. Although many scientific theories are available for analyzing scattering from objects, the process of reducing the scattering is presently as much an art as a science.
Of the three techniques, absorbers have the most attractive features. They have a broad bandwidth, attenuate the return in many directions, and may be used to reduce scattering from an object after the object is designed. In contrast, shaping an object does not reduce the scattering in all directions, may not even be possible once the object is past the design stage, and may not reduce the scattering to desired levels. Impedance loading is inferior because it has a narrow bandwidth, is not usually feasible past the design stage, and is not practical for large reflecting surfaces.
Absorbers have low scattering levels because they convert most of the incident electromagnetic energy into heat and only a small percentage is reflected or transmitted. In the absorber the amount of energy converted into heat (absorbed) depends on the size of the imaginary part of the index of refraction. The higher the imaginary part, the more energy the material absorbs.